Movable tape condenser for electronic generators



M. MARTENOT 2,581,680

MOVABLE TAPE CONDENSER FOR ELECTRONIC GENERATORS Jan. 8, 1952 FiledApril 5, 1948 3 Sheets-Sheet l QR tanks I W X M. MARTENOT 2,581,680

MOVABLE TAPE CONDENSER FOR ELECTRONIC GENERATORS Jan. 8, 1952 FiledApril 5, 1948 3 Sheets-Sheet 2 M. MARTENOT 2,581,680

MOVABLE TAPE CONDENSER FOR ELECTRONIC GENERATORS Jan. 8, 1952 3Sheets-Sheet 5 Filed April 5, 1948 llllllll 1.111] 2 Patented Jan. 8,1952 OFFICE MOVABLE TAPE CONDENSER FOR ELECTRONIC GENERATORS MauriceMartenot, Neuilly-sur-Seine, France Application April 5, 1948, SerialNo. 19,125 In France April 16, 1947 '1 Claims. I

In the electric or electronic musical instruments, the design of theelements ensuring to the performance an easy control of the frequencyvariations from which arise the musical sounds produced by theinstrument is of fundamental importance. As a matter of fact, saidvariations should be obtained while taking into account the gestureconveniences as well from the point of view of the direction andmagnitude of the gestures as from that of their possible velocity andthe inertia toovercome.

Thus, for example, the oscillation generating circuits are usuallydesigned to allow considerable frequency variations (the whole range ofthe musical sounds, if possible) with a minimum of weight and volume ofthe movable members. This is true in the case of the circuits usingheterodyne beats between high frequency oscillators, feedback devices,and circuits using the discharge of gas-filled tubes, which may becontrolled by rods, tapes, wires, etc., acting upon the frequencyvariation elements or constituting, per se, a part of said elements.

The known device which consists in using a tape of which a conductingportion acts as the movable plate of an air condenser presents, inparticular, great advantages.

This invention has for its object improvements in this tape controldevice, particularly in order to ensure to said device a high operationreliability and to obtain a sufficient range of frequency variationswithout excessive motions.

One of said improvements consists in a tape guiding device comprisingshoes on which the tape bears on its face and on its edge, so as todetermine accurately the position of the movable plate constituted bysaid tape with respect to the stationary plate of the condenser.

Another improvement has for its object to reduce the volume of thestationary plates of the condenser while ensuring a satisfactory rangeof frequency variations. It consists in dividing the stationary platesinto successive sections the spacing of which varies according to a lawwhich is discontinuous from each section to the next one, said sectionsbeing parallel connected through fixed condensers suitably selected tocorrect the curve of the capacity variation as the tape moves in thevarious sections.

The stationary plates are preferably constituted by combs which allowsto adjust very accurately the capacity variation for the variouspositions of the tape by slightly twisting the comb teeth. I

Another improvement concerns the tape design proper in the purpose ofobtaining a good surface continuity between the insulating portion andthe conducting portion of said tape, at least in which concerns theparts of the tape which wipe on the guides. According to thisimprovementthe metal is incorporated to the tape by weaving of metallicor metallized wires which may take the place of woof and warp threads ofthe insulating portion without modifying in a prejudical manner itsthickness.

The followin description with reference to the accompanying drawing,given as an illustration which does not limit my invention, will makeeasy to understand how it may be realized, the features which resultfrom the drawing and from the description pertaining, of course, to thescope of said invention.

Fig. 1 is a plan view of a known variable condenser with a movable tape.

Fig. 2 is a similar view and shows a surface guiding according to theinvention.

Fig. 3 is a section of Fig. 2 along line III-III.

Fig. 4 is a front view of portion of the tape shown on the upper part ofFigure 2, the stationary plate assembly of the condenser being removedfor convenience of illustration.

Fig. 5 shows an embodiment of the stationary plates of the condenseraccording to the invention.

Fig. 6 is a wiring diagram showing the connection in the oscillatorcircuit of the various sections of the stationary plate.

Fig. 7 is a front view of a comb-like plate section.

Fig. 8 is a sectional view along line V1IIVIII of a plate sectionshowing also the insulating support.

Fig. 9 shows another way of connecting the condenser plates in theoscillator circuit.

Fig. 10 is a diagram showin the curves of frequency variations which maybe obtained by means of the circuits of Figs. 6 and 9, respectively.

The known movable tape condensers are similar to that shown in Fig. l.The endless tape I is guided on the four rollers 2 and presents aconducting portion a, b, c, 61 shown in heavy line. A ring 3 that theoperator holds in his hand, is used to move the tape between the twostationary plates 4 which together with the metallized portion a, b, c,d of the tape constitute the condenser. This device presents thedrawback of a considerable span of the tape between the outer rollersfrom which may result a lateral variations disturbing the value of thecapacity and affecting the sound qualities. order to overcome saiddrawback, according to my invention, I add to the tape along itseffective travel an accurate guiding device comprising (Figs. 2, 3 and4) shoes against which wipes the tape surface and which causes saidtapes to take an incurved surface in order to ensure the bearing of thetape on said shoes owing to its tightening. As it will be understoodsaid shoes avoid the lateral vibrations of the tape. Its verticalmotions are also avoided by means of another set of shoes 6, which aredisposed under the tape and on which said tape bears on its edge. 7

The known condensers shown in Fig. 1 presents further the drawback thatif it is desired to produce therewith a capacity variation giving afrequency variation corresponding to the scale of the musical intervals,then for the same motion of the tape the capacity variation should beexceedingly small at one end of the condenser and very large at theother end. This leads to give the stationary plates 4 an outline similarto that shown in Fig. l.

The construction of such a device presents the disadvantage of a toolarge size of the plates on the side of the smaller capacity; on theother hand, the capacity to be obtained on the opposite side is suchthat it is no more possible to use air as a dielectric. Moreover the useof a dielectric other than air does not provide a sufficient stabilityof the adjustment.

In order to overcome said disadvantages:

(l) The stationary plates are divided, according to the invention,lengthwise into a number of portions 4a, ib, 4c, 4d (Fig. 5).

(2) Each portion is constituted by two comblike adjustable plates (Figs.'7 and 8) disposed on an insulating base especially shaped to give thedesired curve. However the fine tuning is realized by slightly twistingthe teeth of each comb to bring them more or less away from the tape.

(3) Each elements 4a, 4?), 4c, 4d so constituted is, per'se, independentof any other one. 'As shown in'Fig. 6 said elements are interconnectedin parallel and the whole is also parallel connected to the principaloscillating circuit 0 connected to the anode 8 of the oscillator tube.Fixed capacitors 9 are series connected respectively to the variouselements except that or those of the high-pitches register such as id,said capacitors being suitably designed to correct the aspect of thefrequency variation law and to give it that of the continuous platecondenser of Fig. 1.

In each element the spacing between both stationary plates decreasesaccording to a law adapted to the frequency range which must be coveredby said element but it is possible owing to the division of the elementsand due to link capacitors 9 to obtain convenient intervals (neither toolarge nor too small) at both ends of the element.

According to a well-known arrangement the oscillator comprising theoscillating circuit 0 and the tube 8 shown in Fig. 6 which generateshigh frequency oscillations is coupled with a second oscillator, notshown, and the audio frequency beats thus produced are finallytransmitted to a loud-speaker. It appears from Fig. 6 that in eachelement the end of the stationary plates nearer to the tape, is at asmaller distance from said tape than the adjacent end of the stationaryplates in the adjacent element. For instance the left end of eachstationary plate in the eleferences in the thickness of the tape.

ment to is at a smaller distance from the tape than the right end of thestationary plates in the element ed.

In the portion of the tape condenser which should produce the maximum offrequency variation (high-pitched register), in order to avoid the useof a dielectric other than air it has been found necessary to furthermodify the frequency variation curve by adding an inductance I0 (Fig. 9)of a suitable value to remain sufliciently near to the resonance pointwith the principal oscillating circuit so that in the second part of thetravel of the variable condenser tape the frequency variation obtainedbecomes more considerable, according to the laws concerning the staticcoupled oscillating circuits. So that the value of this circuit shouldbe easily adjusted and to eventually readjust the tuning of thehighpitched register an adjustable or trimming condenser I I is used toshunt said inductance. There may be chosen, e. g., an inductance i ll of10 millihenries shunted by a condenser I l of 15 through 30 cm. for aprincipal oscillating circuit 0 having a capacity of 500 cm. and aninductance of 4 through 5 millihenries, the above values being given, ofcourse, merely as an illustration.

Fig. 10 shows the result obtained with this arrangement. In this figurethe variable condenser capacity is plotted on the ordinate axis, thefrequency being plotted on the abscissa axis. I2 shows the curveobtained without inductance l0 and 33 shows the curve which may berealized with said inductance and which is, as it will be seen, moreextended than the first one on the side of the higher frequencies.

In a condenser of the type described, the variable plate is constitutedby a tape having a conducting portion and an insulating portion.Therefore such a plate was designed by juxtaposing an ordinary tape to aSpangled tape. The juxtaposition was effected either by sewingedgeto-edge, either by means of pieces sticked or sewed on both sides ofthe joint. However such joints are too weak; moreover they cause dif-Even if they are very small, such thickness differences produce a suddenlateral shift of the tape whichever they pass over the guiding shoeswhich results in a sudden capacity variation which does not correspondto the desired curve.

According to my invention, said disadvantage may be avoided as follows:On a warp of insulating thread having the total length of .the tapethere is added only in the portion which is to be conducting warpthreads constituted by conducting wire or metal-lapped thread. The woofis then woven partly in ordinary thread, partly in conducting wire sothat in thickness as well in width, the size of the tape is the same onthe insulating portion, on which the woof threads are interconnected bythe conducting Warp threads, and on the conducting portion. In Figure 4only the metallic warp and woof threads of the conducting portion a, b,c, d of the tape have been shown. A thickness difference may however beadmitted under the condition that it concerns only the tape face remotefrom the guiding shoes 5 of Fig. 2, a width difference being alsoadmitted if it concerns only the tape edge remote from holding shoes 5of Fig. 4.

These conditions may be easily realized in the modern weaving art. Inparticular, it is well known that thick fabrics known, for example, asdouble-stuff, and comprising two woofs and two warps may be obtained. Itis therefore possible for realizing the composite tape to make a thickfabric in the insulating portion then to replace in the metallizedportion certain elements of the thick fabric by metallic woof and warpwires. By suitably choosing the size of the metallic wires, it ispossible to obtain the same thickness and width for both portions.

The purpose of the conducting wire of the woof is to ensure the perfectelectric conductibility of the metallized portion in the case where abreak of continuity would occur in the metallic part of the warp, wireor wires.

It is also to be understood that the embodiments described have beengiven merely as illustrations and that they could be modified,particularly by substitution of technica1 equivalents within the scopeof my invention.

What I claim is:

1. In a hand actuated frequency variation device particularly adapted toelectric musical instruments and having a movable tapewhich is ofinsulating material on a portion of its length and of a metal containingmaterial on the other portion, said tape being associated with astationary plate so as to form therewith a variable condenser, and handactuated means provided on said tape for reciprocating the same, theimprovement wherein said stationary plate comprises a plurality ofelongated metallic elements consecutively arranged in the direction oftheir larger dimension and in a position substantially parallel to saidtape, said elements being curved to provide a variable space betweeneach of said elements and said tape, said elements being furtherpositioned with respect to said tape so that the end of each elementnearer to the tape is at a smaller distance from said tape than theadjacent end of the adjacent element.

2. The improvement of claim 1 comprising further guiding means arrangedalong the path of the tape in front of said elements of the stationaryplate and cooperating with said tape in order to maintain anunchangeable space between its surface and the surface of said elements.

3. The improvement of claim 1 comprising further at least two rollersrespectively arranged near the opposite ends of said stationary platefor supporting said tape and guiding means arranged along the path ofthe tape between said rollers, said guiding means cooperating with saidtape in order to maintain an unchangeable space between the surface ofsaid tape and the surface of said stationary plate.

4. The improvement of claim 1 wherein said tape is an endless band andmeans are provided for guiding said band for longitudinal movement, saidguiding means comprising at least two rollers arranged near the ends ofthe stationary plate, and means acting between said rollers to offsetsaid tape from a straight line path between said rollers, and othermeans acting between said rollers and arranged sidewise with respect tosaid tape to prevent lateral vibrations of said tape in its plane.

5. A frequency variation device according to claim 1 comprising furthermeans for electrically connecting said elongated elements of thestationary plate to an electric oscillator, said means comprising aplurality of fixed condensers, each having one plate connected to one ofsaid elements and the other plate adapted to be connected to saidoscillator, said fixed condensers being adapted to correct the curve ofthe capacity variation of the variable condenser as the tape moves alongsaid elongated elements.

6. A frequency variation device according to claim 1 comprising furthermeans electrically connected to said variable condenser for connectingsaid variable condenser to an electric oscillator, said means comprisingan inductance and a trimming condenser shunting said inductance, saidinductance and. said trimming condenser being adapted to extend thefrequency variation curve on the side of the higher frequencies.

7. A frequency variation device according to claim 1 in which said tapeis formed by an insulating fabric and said metal containing portion ofsaid tape comprises metallic or metallized wires incorporated in saidfabric.

MAURICE MARTENOT.

REFERENCES CITED L The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,543,990 De Forest June 30, 19251,744,004 Hunt Jan. 14, 1930 2,024,558 Wilson Dec. 17, 1935 2,055,719Fisher Sept. 29, 1936 FOREIGN PATENTS Number Country Date 153,300 GreatBritain Feb. 1, 1922 657,919 France Jan. 21, 1929 378,291 Italy Jan. 30,1940

